1. Field of the Invention
This invention relates generally to aqueous lacquers. More particularly, the invention relates to new alkyd resins which may be processed to form aqueous lacquers that are particularly harmless to the environment.
2. Statement of the Prior Art
It is known that alkyd resins having a high hydroxyl number (above 100) and a considerable residual acid number may be used in the production of oven-drying aqueous lacquer systems that are particularly harmless to the environment. To produce alkyd resins such as these, it is standard practice to start with resins having a high acid number, for example in the range from 30 to 180, and to produce the required resins by condensation with substances containing or yielding OH-groups.
British Pat. No. 1,038,696 discloses condensing acidic resins containing polyethylene glycol units by reaction with polyfunctional alcohols, to an acid number below 10. After neutralization with amines for example, alkyd resins produced in this way are self-emulsifying and may be formulated into aqueous lacquers which, unfortunately, do not have the required resistance to water after heating. Another disadvantage of these resins is that incompatibilities often arise at the pigmenting stage causing reduced gloss.
German Pat. No. 2,842,919 discloses hydroxyl-group-containing alkyd resins having an acid number of from 5 to 35 and, more particularly, from 15 to 30, in which it is proposed to esterify an acid-group-containing resin having a starting acid number of from 30 to 180 with glycidol. Although binders having favorable properties may be obtained in this way, the use of glycidol involves problems because this starting material is difficult to obtain and difficult to handle.
Accordingly, U.S. Pat. No. 4,271,051 (and corresponding German Patent Application No. 29 22 370) proposes condensing an acidic resin having an acid number of from 30 to 180 with glycerol to an acid number of from 5 to 35 and more particularly from 15 to 30. After neutralization, the resins may be processed to form clear, solvent-free aqueous preparations. However, the aqueous preparations thus obtained are sensitive to calcium ions which are entrained, for example, during pigmenting with calcites. Another disadvantage of the systems in question is that their resistance to water is still inadequate for numerous applications. Accordingly, there is a need for a new aqueous lacquer system based on a resin having an acid number below 40 and a high OH-number which may easily be produced from readily obtainable starting materials, which may be pigmented with calcite, and which shows the required resistance to water.